This is not new - my guess, it's old. I have noted a few scripts/examples
vulnerable to this previously, and would guess that we are not alone to have
seen it before. "Yet another bad way to code PHP / ASP / Whatever" (TM).

But I never noticed the subtle variant you describe (perhaps since I never
looked for it!), but a much more general/stupid variant:

$var = $_GET["something"];
$_SESSION["$var"] = $var2;

Yes. People do really code like that. Noted that one in some example posted
in some php forum.

Anyway, there are probably a quite a lot of scripts which are vulnerable to
this issue if php.ini is misconfigured (register_globals = on). Then we get
the classic "if value not assigned, request parameter value will be
injected" scenario which allready is known to enable several SQL Injections.